Major oil companies are gearing up to build slurry bubble column reactors (SBCR) to utilize natural gas located in remote areas of the world and to convert it to paraffin wax which will be upgraded to gasoline and Diesel fuels. SBCRs have recently become competitive with traditional fixed bed reactors for converting synthesis gas into liquid fuels. The advantages of the slurry-phase reactor over the fixed bed reactor are well documented. In slurry bubble column reactors, fine powdered catalysts are suspended in the fluid and gas bubbles provide the energy to keep the catalyst mixed. SBCRs have excellent heat and mass transfer characteristics for removal of the heat given off by exothermic reactions and the ability to replace catalyst easily.
The design and scale-up of SBCRs require, among other things, precise knowledge of kinetics, hydrodynamics and mass transfer characteristics over a wide range of operating conditions for reactors with a diameter as large as 7 m and a height of 30 m being built by the oil industry. Models applied to the F-T conversion of synthesis gas in a SBCR require hold-up correlations, diffusivity, mass transfer coefficients and bubble size as inputs. As an input in such a model, eddy diffusivities are measured using computer-aided radioactive particle tracking (CARPT) and mass transfer coefficients in three-phase flows, including fine particles. Hold-up profiles for these reactions have been measured, but coherent flow structures in bubble column reactors in the churn-turbulent regime have not been computed.
Computational fluid dynamics (CFD) is a recently developed tool which can help in the scale up. These multiphase CFD codes with viscosity as an input allow for the computation of hold-up and flow patterns for gas-liquid flow and gas-liquid-solids flow. Based on a kinetic theory model, the hold-up, flow patterns and methanol production in an Air Products/Department of Energy (DOE) Laporte slurry bubble column reactor have been computed and used for modeling new reactor designs. But most modeling studies have not addressed the effect of catalyst size on the performance of the reactor.
An issue of interest to the energy industry is the size of the catalyst used in slurry bubble column reactors. The industry is gearing up to make catalysts for slurry bubble column reactors. Fischer-Tropsch catalysts, such as those used to produce methanol and other liquids from synthesis gas normally are in powder form. Catalyst particles used in most fluidized bed processes are small enough such that external mass transfer and internal diffusion resistance are negligible. The size of the catalyst is typically in the range of 20 μm to 120 μm, although some workers in this area describe the preferred particle size as between 20 and 80 μm. Small particle sizes are needed to carry out the reaction. However, small particles become entrained in the product gas stream and are known to cause liquid product filtration problems. Small particles also cause the formation of the clusters, which give rise to large effective particle sizes and hence poor mass transfer.
The present invention is concerned with optimum particle size, which is the size that provides maximum granular temperature, similar to the experiments for gas-solid systems done at EXXON by Cody in 1996. For this particle size, the heat and the mass transfer coefficients have the highest values. In the present invention the mass transfer coefficient is an input and the mass transfer coefficient is correlated to the fluctuating velocities (granular temperature) as determined by a hydrodynamic model.